The present invention relates generally to signal processing, and more specifically to processing global navigation satellite system signals.
In the processing of global navigation satellite system (GNSS) data, two basic types of measurements are typically used to determine the position of a satellite receiver. One type of measurement is using a coded time mark of the transmission time of satellite signals. The other type of measurement is a phase (difference) measurement, the difference between the phase generated by the satellite oscillator at transmission time and the phase in the receiver's oscillator at receipt time.
With respect to the first type of measurement, a receiver receives signals transmitted by a satellite and, if needed, decodes the signals. Once decoded, the transmission time of the signals are compared to a received time of the signals determined by the receiver. The difference between the received time and the transmitted time is then multiplied by the vacuum speed of light to yield a pseudorange observation.
Along with the transmission time, tables of satellite ephemeris parameters are also typically transmitted by the satellite. With the satellite ephemeris parameters and pseudoranges from a minimum of four satellites, the location of a receiver at or near the earth's surface can be determined. The location is typically accurate to within several meters and often depends on the quality of the geometrical distribution of satellites and the number of satellites.
As described above, the other type of measurement used is phase measurement. The phase measurement is typically the measured difference between the carrier's phase state at transmission time and the phase state of the receiver at reception time. The difference is usually measured in the receiver's hardware utilizing a phase locked loop (PLL).
The precision typically considered for the PLL-based measurements is typically on the order of 0.01 cycles. The wavelengths of GNSS signals (having frequencies identified by identifiers L1 and L2) are approximately 19 and 24 cm, respectively. The expected quality, or accuracy, of the L1 and L2 phase measurements is approximately 2 mm, using the 0.01 cycle expectation.
The increase in phase measurement accuracy over pseudorange accuracy, however, occurs at a price. While pseudoranges are biased by the receiver's clock error, phase measurements are biased by initial phase offset values and an arbitrary cycle count (integer) (i.e., cycle ambiguity). While changes from epoch to epoch in phases are unambiguous as long as a lock on the phase signals is maintained, absolute phase values contain an unchanging ambiguity.
Because the accuracy associated with phase measurements (i.e., typically millimeter accuracy) is more appealing for use in precise positioning applications, there remains a need to utilize these phase measurements better.